The invention relates to a stud enclosure for protecting a stud member of a stud/nut fastening system designed to maintain a removable vessel head against a pressure vessel and to a method of using the stud enclosure to protect the stud from corrosion when the head must be removed. More particularly, the invention relates to a stud enclosure for protecting a stud extending upwardly of a flange of a nuclear reactor pressure vessel (xe2x80x9cRPVxe2x80x9d) when the RPV is submerged in a borated pool of water and the RPV head is removed in the course of a refueling outage or an inspection outage.
Commercial nuclear plants for generating electric power, including pressurized water reactors and boiling water reactors, are taken off-line on a periodic basis and the RPVs are opened in order to refuel or inspect the RPVs. Many steps must be performed on a critical path schedule to permit safe access to the internal regions of a RPV. In accordance with current procedures, up to fifty four or more large, heavy RPV studs fastening the removable RPV head to the RPV, are detensioned by several suspended detensioners operating simultaneously. The RPV nuts, washers and studs are removed and the RPV stud holes are plugged. The RPV head is then moved to a remote stand, the reactor cavity and refueling canal are flooded and the refueling or inspection operations are performed. Upon completion of the refueling or inspection operations, the reactor cavity and refueling canal are drained and the RPV head is then replaced on the RPV flange. After placing the RPV head on the RPV flange, the studs, washers and nuts are replaced and the nuts retensioned. Many of these steps and their auxiliary steps require the use of the plant""s overhead polar crane so that the crane is not then available for other tasks.
The studs are removed from the RPV to prevent corrosion caused by a sequence of events including the exposure of the studs to boron-containing water when the RPV is submerged in a refueling pool and the subsequent exposure of the studs to simultaneous high temperatures and high stresses during on-line operations. Similarly, the stud holes in the RPV flange are plugged to protect them from corrosion.
It is an object of the present invention to provide a stud enclosure for protecting RPV studs extending from a RPV flange from corrosion resulting from submersion in the refueling pool during an off-line operation. It is another object of the present invention to reduce the number of tasks that need to be performed by the plant""s polar crane during an off-line operation.
With these objects in view, the present invention resides in a stud enclosure for protecting a stud upwardly extending from a nuclear reactor pressure vessel (RPV) flange. The stud enclosure includes a cylindrical can having an open end and a capped end. The capped end has an axially extending hole therein and a fastener member extends through the axially extending hole in the capped end. Advantageously, the fastener member can be employed to threadedly engage an axially extending hole in the end of the RPV stud. A seal ring is adjacent the open end of the cylindrical can. Advantageously, the seal ring can form a seal with the RPV flange when the stud enclosure is positioned over a RPV stud. A gas valve is disposed in the capped end of the cylindrical can. Advantageously, the gas valve can be employed to introduce gas into the interior portion of the stud enclosure to raise the interior pressure to about 10 psi (which is equivalent to about 22 feet of water) when the stud enclosure is seated on a RPV flange to resist the head of water in the refueling pool and to retard leaks into the enclosure.
The present invention also involves the method of protecting a stud extending from a nuclear reactor pressure vessel (RPV) flange. In accordance with the practice of this method, a fastener nut and washer are removed from threaded engagement with a stud extending from a RPV flange and through a stud hole of a flange of a removable RPV head supported on the RPV. A stud enclosure is placed over the stud and into physical contact with the RPV flange while the RPV head is supported on the RPV flange. The stud enclosure is fastened to the stud and the internal portion of the stud enclosure is pressurized with a gas, e.g., plant air. Advantageously, the practice of the present invention reduces the demands upon the use of the plant""s polar crane so that the critical path time can be relaxed. Also, the exposure of workers to radiation will be reduced. It has been estimated that up to about seven hours could be saved during an outage. In addition, the practice of the present invention would also save the effort required to handle, clean and re-lubricate the studs and stud holes.